This invention relates to a circuit board comprising a conductive pattern section with a pattern formed on a conductive layer on a substrate and in particular to a circuit board in which bonding areas are defined in a conductive pattern section and an ultrasonic bonding method is used to join bumps of a part mounted on the board.
A circuit board comprising a conductive layer such as copper foil bonded to a substrate and formed in a desired pattern for realizing wiring connection is used as a card-type printed-circuit board used with a large electronic machine. In addition, in recent years, various circuit boards, large and small, have been used diversely as high-density packaging methods of multi-chip mounting, bear chip mounting, etc., have become pervasive. Examples of the circuit boards are a circuit board to which a chip part is directly bonded with face down, a circuit board to which bumps and tabs can be bonded ultrasonically, and a circuit board serving as both a wiring board and a vessel of parts for use with electronic parts.
A circuit board for use with a chip-type surface acoustic wave device will be discussed with reference to FIG. 11 and FIG. 12 as a recent circuit board example dealing with high-density packaging of the prior art.
FIG. 11 is an assembly view to show the structure of a conventional surface acoustic wave device. For purposes of signal input/output, ground connection, and as required, power connection, eight electrodes bumps (simply bumps) 22 each about several ten microns high are installed on one side (rear side in FIG. 11) of a surface acoustic wave chip 21 of a niobium acid lithium crystal, etc., which will be hereinafter called chip 21. A circuit board 23 on which the chip 21 is placed comprises a conductive pattern section 24 with a pattern formed on a conductive layer, and bonding areas 25 are defined in the conductive pattern section 24 as predetermined areas in which the bumps 22 are bonded ultrasonically and secured fixedly. The bonding area 25 is an area comprising a bonding position that the bump 22 directly strikes and a portion where the bump is spread, welded, and joined by ultrasonic bonding. After the chip 21 is placed on the circuit board 23 by ultrasonic bonding, the circuit board 23 is housed in a vessel 26 and sealed by a lid 27. Such a related art example can also be seen in Unexamined Japanese Patent Publication 4-65909.
The circuit board 23 generally comprises metal foil of copper, etc., about 12-75 microns thick deposited as a conductive layer via an adhesive layer about several ten microns thick on an insulating substrate of a resin, ceramics, etc., about 0.2-2.5 millimeters thick, the conductive layer being etched in a desired pattern for forming a plane circuit of the conductive pattern section 24. In recent circuit boards, a conductive layer is etched in a width or space of about several hundred microns for forming a high-precision conductive pattern section.
Ultrasonic bonding for mounting a part with bumps on the circuit board will be discussed with reference to FIG. 12. First, the chip 21 with the bump 22 face down is attracted at the chip of a head section 29 of a ultrasonic bonder 28. Next, each of the bumps 22 is aligned with the center of each bonding area (bonding position) in the conductive pattern section 24 on the circuit board 23, the head section 29 is brought down, and the bumps 22 are pressed against the surface of the conductive pattern section 24. Ultrasound is added to the bumps 22 via the chip 21 from the head section 29 and ultrasonic energy is used to join the bumps 22 and the conductive pattern section 24 for fixedly securing the chip 21 to the circuit board 23.
However, according to the related art, the following problems remain unsolved:
To ultrasonically bond a part provided with bumps onto a circuit board, it is difficult to provide all bumps with the same degree of bonding strength and the bonding strength varies from one bump to another.
To ultrasonically bond the bumps to the bonding areas, essentially it is desirable to apply optimum ultrasonic bonding energy to the bumps in response to the bump shape, the conductive pattern section form containing the bonding areas, and the like. For example, to ultrasonically bond the bumps to the bonding areas of comparatively narrow conductive land portions separated in three directions by the pattern boundary as shown in conductive lands 24a in the conductive pattern section 24 in FIG. 11, it is desirable to apply comparatively strong ultrasonic energy for a long time and enlarge load imposed on the chip.
To ultrasonically bond the bumps to the bonding areas of comparatively wide conductive land portions as shown in conductive lands 24b in the conductive pattern section 24 in FIG. 11, it is not necessary to apply strong ultrasonic energy to the bumps or impose large load thereon; if too strong ultrasonic energy is applied to the bumps, the circuit board or the chip is cracked and broken, etc.
However, on the one hand, usually the shape and area of the conductive pattern section formed in the circuit board vary diversely depending on the electric signal type, current capacity, pattern application, etc.; on the other hand, the ultrasonic energy applied to the chip from the head section of the ultrasonic bonder is distributed only mechanically simply among the bumps by pressing the chip against the conductive pattern section.
It is desirable to apply appropriate ultrasonic energy separately to each of the bumps combined with the bonding areas. In fact, however, joining energy cannot be adjusted. Resultantly, to ultrasonically bond a chip provided with bumps to a circuit board, the energy for joining the bumps and bonding areas becomes too much or too little; the ultrasonic bonding failure rate is high and the joint parts also lack reliability.
Further, taking a flexible manufacturing system or an automatic packaging apparatus with a robot as the recent trend as an example, to use one ultrasonic bonder to ultrasonically bond a chip with bumps to various circuit boards or various parts with bumps to points of a large circuit board in multi-chip mounting, etc., the operation conditions of the ultrasonic bonder must be again adjusted to the optimum bonding conditions each time the circuit board type changes or the conductive pattern section shape or the number of bumps of a part varies; thus there is a serious defect of lacking practicality because the substantial productivity extremely lowers.
Resultantly, to mount a chip with bumps on a circuit board, the ultrasonic bonding involves problems of lacking versatility, poor conductivity, and lacking reliability after joining.
It is therefore an object of the invention to provide a circuit board which enables a chip with bumps to be fixedly secured to bonding areas in a conductive pattern area on the circuit board evenly in high bonding strength.
Other and further objects and new features of the invention will become apparent from the following description of embodiments.
To the end, according to a first aspect of the invention, there is provided a circuit board comprising a conductive pattern section with a pattern formed on a conductive layer provided on a board main body, wherein two or more bonding positions that bumps of a part mounted by ultrasonic bonding strike are set in the conductive pattern section, characterized in that an isolated notch part or recess (for example, an island-like notch or recess pattern surrounded on four sides by the level difference from the conductive layer) is formed in the conductive layer in the proximity of at least one bonding position.
According to a second aspect of the invention, there is provided a circuit board comprising a conductive pattern section with a pattern formed on a conductive layer provided on a board main body, wherein two or more bonding positions that bumps of a part mounted by ultrasonic bonding strike are set in the conductive pattern section, characterized in that in the conductive layer in the proximity of at least one bonding position, a notch part or a recess extending from the margin of the conductive pattern section to the inside thereof and reaching the proximity of the bonding position (for example, a groove-like notch or recess pattern surrounded on three sides by the level difference from the conductive layer) is formed.
In the above-mentioned circuit boards of the invention, the notch part or the recess may be formed extending in a direction crossing the ultrasonically vibrating direction of the ultrasonic bonding.
The notch part or the recess may partially narrow the conductive pattern section (for example, to a constricted shape) to form a narrow pattern part.